1. Field of the Invention
The present invention relates to a one-side spot welding method and a one-side spot welding apparatus that perform welding by causing an electrode to contact a welding portion of work-pieces only from the one side thereof.
2. Description of the Related Art
When joining a plurality of work-pieces (sheet metals, etc., in a tabular shape), a spot welding technique is applied which allows a current to flow through the work-pieces while applying welding pressure to such work-pieces, and which causes metals to be melted by resistive heat generated by the current flow and the pressurization, thereby welding the work-pieces together.
This spot welding is classified into various kinds depending on the differences of the way of causing the welding current to flow, but roughly classified into two kinds; a both-side spot welding which allows a current to flow through the work-pieces while applying pressure from both sides of the work-pieces; and a one-side spot welding which allows a current to flow through the work-pieces while applying pressure from only the one side of the work-pieces.
Since work-pieces are in various shapes, it is physically difficult in some cases to cause electrodes (welding guns) to abut the work-piece from both sides thereof when work-pieces in a complicated shape are a welding target. In this case, the one-side spot welding is inevitably applied.
As explained above, in comparison with the both-side spot welding, the one-side spot welding has a broad range of the welding targets to which the one-side spot welding is applicable. However, since application of pressure is possible only from the one side of the work-piece, it is difficult to apply sufficiently high pressure to the work-piece when joining the work-pieces together. Hence, in comparison with a case in which the work-pieces are joined together by both-side spot welding, gas pockets (gaps) are likely to be produced at the welded portion in joining of the work-pieces together. When the welded portion is solidified without removing the gases in the gas pockets, such gas pockets highly possibly become blow holes conclusively.
JP 2011-31271 A discloses a one-side spot welding technique that causes a welding electrode and a plurality of ground electrodes to abut a work-piece to let a current to flow across both kinds of electrodes.
According to this structure, a plurality of current paths between the welding electrode and the grounding electrodes are formed, and the current density at a portion of the work-piece where each electrode abuts can be dispersed. Accordingly, it becomes possible to prevent the current density from being concentrated at only one abutting location, and the work-piece can be softened entirely. As a result, the whole electrodes appropriately bite into the work-piece, thereby preventing a joining failure.
The technology disclosed in JP 2011-31271 A is, however, not a technology at all to eliminate the generation of the blow holes inherent to the characteristics (the difficulty of applying sufficiently high pressure to a work-piece) of the one-side spot welding, and cannot address the above-explained technical issue.
That is, the technology disclosed in JP 2011-31271 A can prevent a joining failure due to the unevenness of the current density, but cannot prevent a joining failure (the generation of blow holes) due to the insufficient pressure application.
It is an object of the present invention to provide a one-side spot welding method and a one-side spot welding apparatus which can suppress generation of blow holes.